C is a programming language. A matrix is a two-dimensional array - think of a rectangle box of numbers/letters/anything really. A constant is a number that you give a name, and tell the program it can never change it (you'll have things like #define MAXSIZE 12 to say that the max size of something is always 12).
When you declare a matrix, you would have something like:
int my_matrix[10][20];
where 10 indicates the number of rows, and 20 the number of columns in the matrix. int is what is stored in the matrix (lots of integers).
C likes it when you create an array, and you put a constant in the matrix such as
int my_matrix[MAXROW][MAXCOLUMN];
and doesn't like it if you try to think of a number later. In this case, I was reading how many letters were on a line, and trying to tell the matrix that it should have however many letters there were as the column length. There are other reasons why it was difficult for me, but explaining those would take a long time. :)